1. Field of the Invention
The present invention relates to a laser package, in which a laser device or laser devices are placed in a hermetically sealed package. The present invention also relates to a laser module in which the laser package is provided.
2. Description of the Related Art
A laser package which includes a laser device or laser devices and a hermetically sealed package having a package base and a cap with a light transmissive window for emitting light is well known. In such a laser package, the laser device or laser devices are placed in the hermetically sealed package. In the laser package, the laser device or laser devices are normally fixed onto the package base, and a plurality of lead pins including a lead pin for supplying drive current to the laser device or laser devices projects from the package base.
The package base and the cap are fixed to each other by welding. Generally, an Fe-based material such as stainless steel (SUS: steel use stainless), which has low thermal conductivity, is used as a material for the package base and the cap. Such a material is used because of its weldability, its heat transfer suppression characteristic of preventing transfer of welding heat to the laser device or devices or the like. The thermal conductivity of the stainless steel is approximately 16 W/m·K.
Conventionally, relatively low output laser devices, such as an infrared semiconductor laser device with an oscillation wavelength of 980 nm/an output power of 90 mW, have been used as laser devices. The electric/photo-conversion efficiency of the infrared semiconductor laser device is within a range of 40% to 60%. Further, it is estimated that heat from a laser device is within a range of 60 to 135 mW when the output power is at 90 mW.
In recent years, use of higher output laser devices, such an ultraviolet semiconductor laser device, as the laser devices has been considered. The ultraviolet semiconductor laser device is a laser device, such as a GaN-based semiconductor laser device with an output power of approximately 200 mW, for example. The electric/photo-conversion efficiency of the ultraviolet semiconductor laser device is within a range of 20% to 25%. Further, it is estimated that heat from a laser device is within a range of 0.6 to 0.8 W when the output power is at 200 mW. Since the heat generated by the ultraviolet laser device is approximately five to six times of the heat generated by the infrared semiconductor laser device, if a holder made of stainless steel (SUS), which has low thermal conductivity, is used for the ultraviolet laser device, it is impossible to sufficiently radiate heat from the ultraviolet laser device. If the heat is insufficiently radiated from the laser device, the life of the laser package becomes short because of deterioration of the laser device or the like due to the heat.
As a structure for radiating heat from a laser device, a structure illustrated in FIG. 7A has been proposed. In FIG. 7A, a heat radiation plate 130, which has efficient thermal conductivity, is attached to the bottom of a package base 111 of a laser package 110. The heat radiation plate 130 has one insertion hole 131 through which a plurality of lead pins 114 is inserted together. In the structure illustrated in FIG. 7A, the package base 111 and the heat radiation plate 130 are joined together through silicon grease or the like (omitted in FIG. 7A) so as to cause them to be more closely contacted with each other. The package base 111 and the heat radiation plate 130 are closely contacted with each other through the silicon grease or the like to reduce contact resistance therebetween. FIG. 7A is a side view of the laser package. In FIG. 7A, a cross-section of the heat radiation plate 130 is illustrated. Further, a cap 112 of the laser package 110 is illustrated in FIG. 7A.
A heat radiation structure is disclosed in Japanese Unexamined Patent Publication No. 9(1997)-307162. In Japanese Unexamined Patent Publication No. 9(1997)-307162, a laser package is housed in an insulating outer case (7) and a heat radiation plate (8) is provided. The heat radiation plate (8) abuts on a part of the outer circumstance of the bottom of a package base. Further, the heat radiation plate (8) connects a lead pin (11-1) for supplying drive current to a laser device to the insulating outer case (7). Further, a condenser (9) is mounted on the heat radiation plate (8). In this paragraph, the reference numerals in the brackets are reference numerals which are used in Japanese Unexamined Patent Publication No. 9(1997)-307162.
Alternatively, a technique in which a Peltier device (cooling device) 150 is attached to the bottom of a package base 111 of a laser package 110 has been proposed, as illustrated in FIG. 7B. In this technique, insertion holes 151 are formed on the Peltier device 150, and each of a plurality of lead pins 114 is inserted into respective insertion holes 151.
Although there are some exceptions, materials with efficient thermal conductivity tend to have efficient electrical conductivity. In the heat radiation structure illustrated in FIG. 7A, the heat radiation plate made of Cu, Al or the like is used. Therefore, it is necessary to form an opening of the insertion hole on the heat radiation plate so that the diameter of the insertion hole is sufficiently large, thereby preventing the plurality of lead pins from contacting the inner wall of the insertion hole. The plurality of lead pins should not contact the inner wall of the insertion hole to prevent short circuit between the lead pins. Therefore, in a conventional structure, only the outer circumference portion on the bottom of the package base abuts on the heat radiation plate. Hence, it is difficult to cause the laser package and the heat radiation plate to contact with each other at a sufficiently large contact area. Further, there is a tendency that temperature at a central portion on the bottom of the package base, surrounded by the plurality of lead pins, becomes higher than temperature at the outer circumference on the bottom of the package base. However, in the conventional structure, it is impossible to cause the high temperature portion at the center to contact with the heat radiation plate. As described above, the heat radiation efficiency of the heat radiation structure illustrated in FIG. 7A is low. Hence, the heat radiation structure is insufficient to cope with use of a high output laser device, such as a GaN-based laser device.
In the heat radiation structure disclosed in Japanese Unexamined Patent Publication No. 9(1997)-307162, the temperature at the center of the bottom of the package base becomes particularly high. However, it is impossible to attach the heat radiation plate (8) to the center of the bottom of the package base (reference numeral (8) is a reference numeral in Japanese Unexamined Patent Publication No. 9(1997)-307162). This condition is the same as the condition in the heat radiation structure illustrated in FIG. 7A.
In the heat radiation structure illustrated in FIG. 7B, the Peltier device is attached to the bottom of the package base. Therefore, it is possible to more efficiently radiate heat than a structure using a heat radiation plate. However, in the heat radiation structure illustrated in FIG. 7B, it is necessary to form an opening of the insertion hole on the Peltier device so that the diameter of the insertion hole is sufficiently large, thereby preventing the lead pin from contacting the inner wall of the insertion hole. The lead pin should not contact the inner wall of the insertion hole to prevent short circuit with a wire in the Peltier device. In the heat radiation structure illustrated in FIG. 7B, the temperature at the center of the bottom of the package base becomes particularly high. However, it is impossible to cause a central portion on the bottom of the package base and the Peltier device to efficiently contact with each other. This condition is the same as the condition in the heat radiation structure illustrated in FIG. 7A.
If the diameter of a lead pin is larger, heat may be efficiently radiated from the lead pin. However, in such a case, it is necessary to use a special package, and the cost of the laser package becomes high.
Alternatively, a heat radiation plate or the like may be attached to the side and/or the upper surface of a laser package to increase a heat radiation area. However, even if the heat radiation plate or the like is attached to a position which is apart from a high heat generation portion at the center of the bottom of the package base, it is impossible to achieve an efficient heat radiation characteristic. Especially, when the package is made of a low thermal conductivity material, such as stainless steel, it is impossible to achieve an efficient heat radiation characteristic because the package per se has substantial resistance.